1. Field of the Invention
The present invention relates to a solid oxide fuel cell device. More particularly, the invention relates to a solid oxide fuel cell device which has a solid oxide fuel cell aiming at miniaturization and reduction in profile by employment of a simple structure which does not require sealing, and in which a cathode electrode layer (hereinafter called a “cathode layer”) and an anode electrode layer (hereinafter called an “anode layer”) are formed on a solid oxide substrate; which can generate electric power through utilization of flameless combustion based on catalytic oxidation of fuel, and which is convenient in handling, including carriage on the road and transport.
2.Description of the Related Art
Conventionally, fuel cell devices have been developed and put into practice as low-pollution electric power generation means to replace thermal power generation, or the like; or as an electric power source for electric vehicles, that replaces internal combustion engines whose fuel is gasoline, or the like. Furthermore, an attempt has been made to utilize the fuel cell device as a power source of a personal computer, and the like. A number of studies on the fuel cell have been conducted with an aim of increasing efficiency and attaining cost reduction.
Fuel cell devices employ various power generation types. These power generation types include a fuel cell device employing a solid electrolyte. An example of this fuel cell device employs a fired member, which is made of fully stabilized zirconia doped with yttria (Y2O3), as an oxygen-ion-conducting solid oxide substrate. A cathode layer is formed on one surface of the solid oxide substrate, and an anode layer is formed on the opposite surface thereof. The fuel cell is configured such that oxygen or an oxygen-containing gas is supplied to the cathode layer side, and a fuel gas, such as methane, is supplied to the anode layer side.
In this fuel cell device, oxygen (O2) supplied to the cathode layer is ionized into oxygen ions (O2−) at the boundary between the cathode layer and the solid oxide substrate. The oxygen ions are conducted through the solid oxide substrate into the anode layer, where the ions react with the gas, such as methane (CH4), having been supplied to the anode layer, thereby producing water (H2O), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO). Through this reaction, the oxygen ions emit electrons. As a result, a potential difference arises between the cathode layer and the anode layer. Hence, when a lead wire is connected to the cathode layer and to the anode layer, electrons in the anode layer flow into the cathode layer side via the lead wire, whereby the fuel cell generates electric power. Meanwhile, the operating temperature of the fuel cell device is approximately 1,000° C.
However, this type of fuel cell device must be provided with separate chambers, one being an oxygen or oxygen-containing gas supply chamber on the cathode layer side, and the other being a fuel gas supply chamber on the anode layer side. Furthermore, since the fuel cell device is to be exposed to an oxidizing atmosphere and to a reducing atmosphere at high temperatures, enhancement of durability of the fuel cell has encountered difficulty.
Meanwhile, a fuel cell device of the following type has been developed. A cathode layer and an anode layer are disposed on opposing surfaces of a solid oxide substrate; and an electromotive force is generated between the cathode layer and the anode layer by means of placing the fuel cell in a mixed fuel gas in which, e.g., methane gas and oxygen gas are mixed. The principle of electromotive force generation between the cathode layer and the anode layer for this type of fuel cell device is the same as that for the foregoing separate chamber fuel cell device. However, since the entire fuel cell can be brought into substantially a single atmosphere, the fuel cell device can be configured as a single-chamber type to which a mixed fuel gas is supplied, thereby enhancing the durability of the fuel cell.
However, in view that even the single-chamber fuel cell device must be operated at high temperatures of approximately 1,000° C., there arises a risk of explosion of the mixed fuel gas. Here, if the oxygen concentration is reduced to a level lower than the ignitability limit in an attempt to avoid the risk, there arises the problem that carbonization of the fuel, such as methane, progresses and the cell exhibits degraded performance. To this end, there has been developed a single-chamber fuel cell device which can utilize a mixed fuel gas of an oxygen concentration level at which progress of carbonization of the fuel can be prevented while explosion of the mixed fuel gas is also prevented.
The foregoing fuel cell device is a type which comprises a fuel cell housed in a chamber having a sealing structure. Meanwhile, there has been proposed a system having a configuration in which a solid oxide fuel cell is disposed in a flame or in the vicinity thereof, thereby maintaining the solid oxide fuel cell at its operating temperature by the heat of the flame, to thus generate electric power such as disclosed in Japanese unexamined patent Hei 6-196176. The configuration of this electric power generation system is illustrated in FIG. 4.
A fuel cell of the electric power generation system illustrated in FIG. 4 comprises a tubular member formed from a solid oxide substrate 1 of zirconia; a cathode layer 2 which is formed on an inner side of the tubular member and which serves as an air electrode; and an anode layer 3 which is formed on an outer side of the tubular member and which serves as a fuel electrode. The solid oxide fuel cell with the solid electrolyte is disposed in a state of exposing the anode layer 3 to a reducing flame portion of a flame “f” which is given from a combustion unit 4 to which fuel gas is supplied. When this arrangement is adopted, radical components, and the like, present in the reducing flame can be utilized as fuel; and air is supplied to the cathode layer 2 inside the tubular member by means of convection or diffusion. As a result, the cell generates electric power as a solid oxide fuel cell.
Meanwhile, in contrast to the related-art solid oxide fuel cell devices, the foregoing single-chamber fuel cell device obviates the need of strict separation between the fuel and the air. However, the single-chamber fuel cell device inevitably employs a hermetically-sealed structure. In addition, the electromotive force is increased by means of stacking a plurality of planar solid oxide fuel cells with use of an interconnect material having thermal resistance and high electrical conductivity, thereby enabling operation at high temperature. As a result, the single-chamber fuel cell device employing the planar solid oxide fuel cells has a large-scale configuration, thereby posing a problem of being costly.
In addition, in actuation of the single-chamber fuel cell device, temperature is gradually raised to a high level, thereby preventing cracking of the solid electrolyte fuel cell. Accordingly, this type of fuel cell device requires a long time until power generation is started, and requires a lot of labor in operation.
In contrast, the tubular solid electrolyte fuel cell illustrated in FIG. 4 adopts a mode of directly utilizing a flame. Accordingly, this type of fuel cell device is characterized by being an open type, and not requiring housing in a hermetically-sealed container. Thus, by virtue of shortening a period of time required for starting power generation and having a simple structure, this fuel cell device can be advantageous in terms of miniaturization, weight reduction, and cost reduction. Since the device utilizes a flame directly, the fuel cell device can be incorporated in a general combustion apparatus or an incinerator. Therefore, the device is expected to be utilized as an electric power supply device.
However, in this type of fuel cell device, since the anode layer is formed on the outer surface of the tubular solid oxide substrate, radical components produced by means of the flame are not primarily supplied to a lower half of the anode layer. Thus, utilization of the entire surface of the anode layer formed on the outer surface of the tubular solid oxide substrate is inhibited. Therefore, electrical generation efficiency has been low. Furthermore, since the solid oxide fuel cell is heated directly by a flame and unevenly, there arises a problem that cracks are easily produced by rapid temperature changes.
To this end, there has been proposed an electric power generation system which adopts a solid oxide fuel cell device of a type directly utilizing a flame produced by combustion of fuel. The solid oxide fuel cell device serves as convenient electric power supply means, which is configured such that a flame exposes the entire surface of an anode layer formed on a planar solid oxide substrate, thereby attaining enhancement in durability, enhancement in electric power generation efficiency, miniaturization, and cost reduction such as disclosed in US 2004/0086761 A1.
An electric power generation system which adopts the proposed solid oxide fuel cell device is illustrated in FIG. 5. In FIG. 5, elements identical with those of the electric power generation system illustrated in FIG. 4 are assigned the same reference numerals. A solid oxide fuel cell C employed in the electric power generation system comprises a solid oxide substrate 1 which is formed into a flat planar shape; a cathode layer 2 formed on one surface of the substrate; and an anode layer 3 formed on the surface opposite the one surface. The cathode layer 2 and the anode layer 3 are disposed so as to oppose each other across the solid oxide substrate 1.
An electric power generation system is formed with use of the thus-configured solid oxide fuel cell C. The cell C is disposed above the combustion unit 4, to which fuel gas is to be supplied, with the anode layer 3 of the fuel cell C facing downward, thereby exposing the anode layer 3 to the flame “f” produced by the fuel, to thus generate electric power. The combustion unit 4 is supplied with fuel to be combusted and oxidized while producing a flame. Phosphorus, sulfur, fluorine, chlorine, and compounds thereof may also be used as the fuel. However, organic substances requiring no waste gas treatment are preferably used. Examples of such organic fuels include gases, such as methane, ethane, propane, and butane; gasoline-based liquids, such as hexane, heptane, and octane; alcohols, such as methanol, ethanol, and propanol; ketones, such as acetone; various other organic solvents; food oil; kerosene; paper; and wood. Among them, the gases are particularly preferable.
The flame may be a diffusion flame or a premixed flame. However, since the diffusion flame is unstable, and is likely to incur degradation of the function of the anode layer due to production of soot, the premixed flame is more preferable. The premixed flame is stable, and, in addition, easy in adjustment of flame size. Furthermore, production of soot by the premixed flame can be prevented by means of adjusting the fuel concentration.
Since the solid oxide fuel cell is formed into a flat planar shape, the flame “f” given from the combustion unit 4 can be directed uniformly on the anode layer 3 of the solid oxide fuel cell C, thereby attaining uniform application of the flame “f” as compared with the solid oxide cell of a tubular shape. Furthermore, since the anode layer 3 is disposed facing the flame “f” side, hydrocarbons, hydrogen, carbon monoxide, radicals (OH, CH, C2, O2H, CH3), and the like present in the flame can be readily utilized as fuel for electric power generation on the basis of oxidation reduction reactions. In addition, since the cathode layer 2 is exposed to an oxygen-containing gas, such as air, the cathode layer 2 can readily utilize oxygen. Furthermore, when the oxygen-containing gas is blown toward the cathode layer 2, the cathode layer side can be brought into an oxygen-rich state more efficiently.
The electric power generated by the solid oxide fuel cell C is extracted via a lead wire L1 extending from the cathode layer 2 and a lead wire L2 extending from the cathode layer 3. As the lead wires L1 and L2, wires made of platinum or a platinum-containing alloy, which are heat-resistant, are employed.
As described above, a proposed electric power generation system of chamber type requires an electric furnace for raising the temperature of a solid oxide fuel cell to an operating temperature, a supply unit for supplying a fuel gas, and oxygen or air, and the like; hence, the system itself becomes complicated and voluminous. Accordingly, the chamber-type system cannot be carried by a person as an electric power generation system.
In contrast, the electric power generation system proposed herein and adopting a solid oxide fuel cell—which utilizes a flame directly—requires a combustion unit for combusting fuel and producing a flame. For instance, by employing a flame of a candle, cigarette lighter as the combustion unit, a compact electric power generation system which is small and lightweight can be realized. However, since utilizing a flame is required for providing high temperature enough for the solid oxide fuel cell to generate electric power, the power generation system itself encounters difficulty in practical usage. That is, because of the flame being not stabilized during the system being carried outside or being used under mobile condition, which ends up an unstable operation of the electric power generation.